(Anagram48 posted questions about fabric marbling in introductions. I'm answering here so more seekers may find answers) The "fixative" for all marbling on both paper and fabric that keeps the pattern from slipping off the surface is the alum mordant. Use 4 TB per gallon to soak prewashed fabrics for about 20 minutes. Fabric pigments require an additional step in order for fabric to be permanent and washable. With almost all fabric pigments, you should iron from the back with setting at cotton for several minutes, and if possible, delay washing for several days to several weeks. If marbling with tube acrylics on fabric, you technically don't need to heat set, but the colors are more permanent if you wait 2 weeks before washing. I always heat set tube acrylics as well, for increased launderability, and because I rarely have 2 weeks before a show to wait on washout. You mentioned Setasilk or Pebeo Soie. Those fabric paints fall into the type of fabric pigments that require heat setting before post wash. Some fabric pigments may offer an additive called an air cure catalyst, but I don't recommend adding this to paints you are marbling with, since it would be a wild card as to whether the paints would behave the way you want for marbling. Golden acrylics have what they call GAC 900 which is an extender to give their paints a softer hand on fabric, but it also makes the colors spread wildly, in my experience. You can sometimes slow down the spread of a color by just adding distilled water, or adding pure pigment of the same hue, but the amount of dispersing agent (ox gall equivalent, like Photo Flo) in various brands of pure pigment varies. Once you get to know how much various paints spread, you can use a slow spreading one added to a fast spreading one or visa versa in order to achieve a balance. A word of caution, especially when marbling on methyl cellulose--you want to make sure to soak and rinse/wash your fabrics sufficiently after heat setting to be sure all alum is removed, or it will dry rot your fabric and make it tear easily. Rolled hems of scarves are particularly prone to holding on to alum after a casual rinse. Carageenan is lighter, not as thick, as methyl cellulose, so rinses out easier. I'm a studio artist in The Plains, VA and would be happy to talk with you sometime if you have more questions about fabric marbling. Good luck! Enid Adams